This invention relates to projection lense, and more particularly, relates to lenses designed to project an enlargement of an image on a cathode ray tube (CRT) such as a phosphor screen of a television set. Such lenses are described in published European Application Nos. 0,135,215 and 0,135,216.
In projection television sets the image may be projected on to a translucent screen from the opposite side to the observer, the CRT and lens being behind the translucent screen and within a free standing cabinet, the front of which comprises the translucent screen. It is desirable to reduce the depth of the cabinet as much as possible and at least below a value such that the cabinet can easily pass through ordinary living room doors. Folding mirrors are usually used within the cabinet to reduce the depth. If the projection distance, that is the distance between lens and translucent screen, is reduced, then the number of internal mirrors can be reduced, but this means that the lens must project at larger field angles. Wide angle projection lenses, covering .+-.35 degrees or greater, are usually more complex than those for standard projection angles of about .+-.23 degrees.
In colour projection television systems using three cathode ray tubes each with its own lens, it is often not necessary to correct the chromatic aberration of each lens due to the limited spectral bandwidth of each CRT, thus simplifying the lens design to some extent. The use of aspheric surfaces on some lens elements can also reduce the number of elements in the lens. Such designs are disclosed in U.S. Pat. Nos. 4,300,817 and 4,348,081. However, the types of lenses shown in these specifications are not suitable for very wide angle projection systems as the optical performance at large field angles is very poor, both as regards light transmission, and also resolution. The main cause of this poor performance at large field angles is the steep curvature of the field flattening lens adjacent to the CRT faceplate.
In British Patent Application Nos. 2,091,898A and 2,119,113A, and in European Patent Application Nos. 0,135,215 and 0,135,216, the optical problems caused by the field flattening lens are largely solved by using a CRT having a faceplate which is concave towards the projection lens.